1. Field of the Invention
The present invention relates generally to prosthetic replacements of joints and, more specifically, to an implantable patello-femoral joint prosthesis and method for its insertion.
2. Description of Related Art
Although joints of the human body are miraculous mechanical devices often lasting a lifetime with no added lubrication or service, our joints, like any mechanical or biological structure, are subject to certain failures. The joints are naturally lubricated and cushioned by synovial membranes and cartilages so that they are normally subject to little wear. Part of this apparent lack of wear is doubtless due to the living tissue's ability of regeneration and self-repair. Sometimes the body's immune system goes awry and attacks a joint, thereby damaging it irreversibly (rheumatoid arthritis). Sometimes old age and general wear and tear catches up with the joint's biological repair system (osteoarthritis). Sometimes a sharp blow or overextension of the joint results in mechanical damage that cannot be repaired by the normal healing process.
The knee joint is a frequent place for joint damage, and inability to walk normally is a frequent result of such damage. The knee is a common source of problems because the joint has an unusually large range of motion and bears half the weight of the entire body. A primary knee movement is the bending and straightening of the leg in which a lower part of the leg (tibia and fibula bones) flex in relation to an upper part of the leg (femur bone). The knee joint flexes over almost 180 degrees from a kneeling position, where the upper and lower leg are almost parallel to each other, to a straight position, where the upper and lower leg form essentially a straight line. The knee joint can also accommodate a certain amount of rotary motion in which the lower leg rotates a few degrees in relation to the upper leg.
This wide range of motion requires extensive contact surface between the femur and the tibia. The joint is rather loosely held together by tendons and ligaments to permit such a wide range of motion. A front-facing side of the knee joint is protected by a separate knee cap (patella) which is held in place by ligaments and slides over a femoral joint surface as the knee bends. The patella and its ligaments are mechanically involved in joint extension. If any of the joint surfaces (femoral surface, patellar surface, or tibial surface) becomes damaged or roughened, the knee joint will not operate properly.
A common problem is damage to the patello-femoral joint so that free motion of the patella is inhibited and painful. This "runner's knee" can make normal joint movement almost impossible. At one time, before the mechanical and protective functions of the patella were understood, the patella was simply removed in an attempt to cure patello-femoral problems.
Today a variety of prosthetic replacements have been developed for different joint surfaces of the knee joint. In extreme cases the entire joint can be replaced with a prosthetic device. However, such surgery naturally requires a considerable time for recovery. In less extreme cases it may be advantageous to replace only the damaged part of the joint. The present invention is concerned with such a replacement for the patello-femoral joint. This type of knee surgery is less drastic than a complete replacement of the knee. It is designed for patients whose main problems involve only the patello-femoral part of the knee and is directed to providing a smooth replacement surface on the femur on which the patella "rides."
There are a number of prior art devices which have attempted to solve the problem of replacing the trochlear groove, or in some way attempted to solve problems due to improper interaction of the patella and femur. U.S. Pat. No. 3,806,961 to Muller shows a prosthesis in which an annular sector having a guide groove is implanted into the end of the femur. A raised arcuate runner member is implanted into the patella so that the patella can slidably move in the guide groove with the runner member acting as a bearing surface. The prostheses are attached to the bone surfaces by bone cement and by pin-like protrusions that extend into channels cut into the bone. A basic problem with bone cement is that it is not always permanent. A layer of inflammatory tissue forms between the bone and the cement so that the implant eventually loosens.
U.S. Pat. No. 3,878,566 to Bechtol discloses another patellar prosthesis. Here a femoral implant bears a more or less acute groove, and the patellar component bears a somewhat crest-like ridge projection that rides in the groove. Again, the devices are fixed in place with bone cement, although they also bear ridged pegs designed to allow improved bone adhesion. U.S. Pat. No. 4,007,495 to Frazier uses a slightly different approach. The patellar component rides in a femoral groove, but the system is also equipped with a femoral projection that engages a slot in the patellar implant. While this structure prevents separation of the patella from the femur, it also greatly restricts movement of the patella and may result in unnatural joint action. Again, this prosthesis is attached by bone cement and ridged pegs.
U.S. Pat. No. 4,151,615 to Hall provides a femoral component and a patellar component that more closely approximate natural patello-femoral joint motion. However, this device is also designed to rely on bone cement and pegs for adherence to the bones of the joint. U.S. Pat. No. 4,838,891 addresses the adhesion problems of prior prostheses by providing a two-part system where an anchoring device is inserted during a first operation. After the healing process fixes the anchoring device firmly in place, a second operation inserts the weight-bearing part of the prosthesis which engages the anchoring device. However, this system requires multiple operations even to insert a patello-femoral prosthesis-a prosthesis designed to be used in relatively uncomplicated cases.